The overall goal of this research is to understand the regulation of sperm function by calcium, cyclic nucleotides and ion channels at the molecular level. This proposal focuses on the role of a sperm tail ion channel (CatSper1) that is specifically required for male fertility. The CatSper1 channel that we recently cloned from humans and mice is expressed only in the testes. The protein in sperm is strikingly localized in the principal piece membrane. Mice deficient in CatSper1 show no phenotype except that males are infertile due to reduced sperm motility. cAMP-induced calcium influx into sperm is deficient in CatSper1-null mice. Because of its restricted protein expression in the sperm tail and because of its restricted knockout phenotype in male fertility, CatSper1 represents an excellent target for a safe, non-hormonal, male contraceptive. Our preliminary results indicate that CatSper1 is associated with other proteins to achieve its cAMP-activated signaling. We propose to identify the proteins associated with CatSper1. We will achieve two specific aims: 1) To purify the protein complex containing CatSper1 using affinity purification, and 2) To clone the interacting proteins and test the protein interaction using immunoprecipitation and immunostaining. The interacting proteins discovered from this study will be essential for the functional expression of CatSper1. Such an expression system will allow us to study the regulation of sperm function by ion channels at the molecular level. It may also lead to the development of a safe and efficient male contraceptive.